The present invention is directed to improved ink jet printing. More specifically, the present invention is directed to aqueous ink compositions which exhibit good performance in ink jet printing processes. One embodiment of the present invention is directed to a process which comprises incorporating into an ink jet printing apparatus an ink composition which comprises water, a colorant, and a polymer of the formula ##STR2## wherein m, n, x, and y are each integers representing the number of repeat monomer units, and wherein the ratio of x:y is from about 10:90 to about 90:10, and causing droplets of the ink to be ejected in an imagewise pattern onto a recording sheet. Another embodiment of the present invention is directed to a process for reducing stitch mottle in ink jet printing which comprises (a) incorporating into an ink jet printing apparatus an ink comprising water, a colorant, and a polymer of the formula ##STR3## wherein m, n, x, and y are each integers representing the number of repeat monomer units, and wherein the ratio of x:y is from about 10:90 to about 90:10 in an amount effective to reduce stitch mottle, (b) causing a first set of droplets of the ink to be ejected in an imagewise pattern onto a recording sheet in a first swath, and (c) causing a second set of droplets of the ink to be ejected in an imagewise pattern onto a recording sheet in a second swath adjacent to the first swath, wherein stitch mottle between the first swath and the second swath is reduced. Yet another embodiment of the present invention is directed to a process for reducing wet smear in ink jet printing which comprises (a) incorporating into an ink jet printing apparatus an ink comprising water, a colorant, and a polymer of the formula ##STR4## wherein m, n, x, and y are each integers representing the number of repeat monomer units, and wherein the ratio of x:y is from about 10:90 to about 90:10, and (b) causing droplets of the ink to be ejected in an imagewise pattern onto a recording sheet, said polymer being present in the ink in an amount effective to reduce wet smearing of the ink on the substrate. Still another embodiment of the present invention is directed to a process for reducing edge raggedness in ink jet printing which comprises (a) incorporating into an ink jet printing apparatus an ink comprising water, a colorant, and a polymer of the formula ##STR5## wherein m, n, x, and y are each integers representing the number of repeat monomer units, and wherein the ratio of x:y is from about 10:90 to about 90:10, and (b) causing droplets of the ink to be ejected in an imagewise pattern onto a recording sheet to form an image, said polymer being present in the ink in an amount effective to reduce edge raggedness of the ink image on the substrate.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, the system is much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
Another type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known and are described in, for example, U.S. Pat. No. 4,601,777, U.S. Pat. No. 4,251,824, U.S. Pat. No. 4,410,899, U.S. Pat. No. 4,412,224, and U.S. Pat. No. 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
Acoustic ink jet printing processes are also known. As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Thus, when an acoustic beam impinges on a free surface (i.e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which it exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of input power. These principles have been applied to prior ink jet and acoustic printing proposals. For example, K. A. Krause, "Focusing Ink Jet Head," IBM Technical Disclosure Bulletin, Vol. 16, No. 4, September 1973, pp. 1168-1170, the disclosure of which is totally incorporated herein by reference, describes an ink jet in which an acoustic beam emanating from a concave surface and confined by a conical aperture was used to propel ink droplets out through a small ejection orifice. Acoustic ink printers typically comprise one or more acoustic radiators for illuminating the free surface of a pool of liquid ink with respective acoustic beams. Each of these beams usually is brought to focus at or near the surface of the reservoir (i.e., the liquid/air interface). Furthermore, printing conventionally is performed by independently modulating the excitation of the acoustic radiators in accordance with the input data samples for the image that is to be printed. This modulation enables the radiation pressure which each of the beams exerts against the free ink surface to make brief, controlled excursions to a sufficiently high pressure level for overcoming the restraining force of surface tension. That, in turn, causes individual droplets of ink to be ejected from the free ink surface on demand at an adequate velocity to cause them to deposit in an image configuration on a nearby recording medium. The acoustic beam may be intensity modulated or focused/defocused to control the ejection timing, or an external source may be used to extract droplets from the acoustically excited liquid on the surface of the pool on demand. Regardless of the timing mechanism employed, the size of the ejected droplets is determined by the waist diameter of the focused acoustic beam. Acoustic ink printing is attractive because it does not require the nozzles or the small ejection orifices which have caused many of the reliability and pixel placement accuracy problems that conventional drop on demand and continuous stream ink jet printers have suffered. The size of the ejection orifice is a critical design parameter of an ink jet because it determines the size of the droplets of ink that the jet ejects. As a result, the size of the ejection orifice cannot be increased, without sacrificing resolution. Acoustic printing has increased intrinsic reliability because there are no nozzles to clog. As will be appreciated, the elimination of the clogged nozzle failure mode is especially relevant to the reliability of large arrays of ink ejectors, such as page width arrays comprising several thousand separate ejectors. Furthermore, small ejection orifices are avoided, so acoustic printing can be performed with a greater variety of inks than conventional ink jet printing, including inks having higher viscosities and inks containing pigments and other particulate components. It has been found that acoustic ink printers embodying printheads comprising acoustically illuminated spherical focusing lenses can print precisely positioned pixels (i.e., picture elements) at resolutions which are sufficient for high quality printing of relatively complex images. It has also has been discovered that the size of the individual pixels printed by such a printer can be varied over a significant range during operation, thereby accommodating, for example, the printing of variably shaded images. Furthermore, the known droplet ejector technology can be adapted to a variety of printhead configurations, including (1) single ejector embodiments for raster scan printing, (2) matrix configured ejector arrays for matrix printing, and (3) several different types of pagewidth ejector arrays, ranging from single row, sparse arrays for hybrid forms of parallel/serial printing to multiple row staggered arrays with individual ejectors for each of the pixel positions or addresses within a pagewidth image field (i.e., single ejector/pixel/line) for ordinary line printing. Inks suitable for acoustic ink jet printing typically are liquid at ambient temperatures (i.e., about 25.degree. C.), but in other embodiments the ink is in a solid state at ambient temperatures and provision is made for liquefying the ink by heating or any other suitable method prior to introduction of the ink into the printhead. Images of two or more colors can be generated by several methods, including by processes wherein a single printhead launches acoustic waves into pools of different colored inks. Further information regarding acoustic ink jet printing apparatus and processes is disclosed in, for example, U.S. Pat. No. 4,308,547, U.S. Pat. No. 4,697,195, U.S. Pat. No. 5,028,937, U.S. Pat. No. 5,041,849, U.S. Pat. No. 4,751,529, U.S. Pat. No. 4,751,530, U.S. Pat. No. 4,751,534, U.S. Pat. No. 4,801,953, and U.S. Pat. No. 4,797,693, the disclosures of each of which are totally incorporated herein by reference. The use of focused acoustic beams to eject droplets of controlled diameter and velocity from a free-liquid surface is also described in J. Appl. Phys., vol. 65, no. 9 (May 1, 1989) and references therein, the disclosure of which is totally incorporated herein by reference.
U.S. Pat. No. 5,750,592, U.S. Pat. No. 5,623,296, U.S. Pat. No. 5,141,556, U.S. Pat. No. 5,160,372, U.S. Pat. No. 5,169,438, U.S. Pat. No. 5,180,425, U.S. Pat. No. 5,205,861, U.S. Pat. No. 5,221,334, U.S. Pat. No. 5,356,464, U.S. Pat. No. 5,555,008, U.S. Pat. No. 5,580,373, and U.S. Pat. No. 5,648,405, the disclosures of each of which are totally incorporated herein by reference, disclose ink compositions containing one or more of the following materials: SILWET.RTM. L-77, SILWET.RTM. L-7600, SILWET.RTM. L-7604, SILWET.RTM. L-7607. These materials are polyethylene oxide modified polydimethylsiloxane polymers.
U.S. Pat. No. 5,714,538 (Beach et al.) and U.S. Pat. No. 5,719,204 (Beach et al.), the disclosures of each of which are totally incorporated herein by reference, discloses polymeric dispersants used in formulating aqueous ink compositions. The dispersants are graft copolymers comprising a hydrophilic polymeric segment, a hydrophobic polymeric segment incorporating a hydrolytically-stable siloxyl substituent, and a stabilizing segment, such as a reactive surfactant macromer, a protective colloid monomer, or a non-siloxyl hydrophobic monomer.
U.S. Pat. No. 5,486,549 (Itagaki et al.), the disclosure of which is totally incorporated herein by reference, discloses a water-based printing ink composition exhibiting excellently low foaming behavior with sustainability even under adverse conditions of high temperature and intense shearing force encountered in the printing works. The printing ink composition comprises, besides an organic polymer as the binder resin in the form of an aqueous solution or emulsion and a coloring agent, e.g., dyes and pigments, a silicone-based defoaming composition comprising a polyoxyalkylene-modified polydiorganosiloxane, polydimethylsiloxane, finely divided silica filler and organopolysiloxane resin mainly or solely consisting of monofunctional organosiloxane units and tetrafunctional siloxane units each in a specified weight proportion.
Ink jet printing frequently is performed with a printhead which has a width less than that of the substrate to be printed. The printhead typically traverses across the substrate in a processing direction to print a swath, and the substrate is then advanced in a direction perpendicular or transverse to the processing direction, enabling the printhead to traverse the substrate again and print another swath adjacent to the already-printed swath. Stitch mottle is a phenomenon observed in ink jet printing when nonuniform unprinted areas (appearing, for example, white when printing on white paper) occur where the swaths meet during the printing process. During the printing process, the ink appears to "pull back" from the stitch/swath edges or lines before the ink dries, resulting in nonuniform images and image defects, especially in solid image areas, which appear as unprinted (white, for example, on white paper) lines running across the image in a direction parallel to the swath. The problem is particularly prominent when pigment colorants are employed in the ink. Another problem often encountered in ink jet printing processes is wet smear. Wet smear occurs when an image exhibits smearing when subjected to the action of a wet, dynamic, abrasive physical contact, such as a wetted thumb dragged across the image, a felt tipped marker dragged across the image, or the like. Yet another problem often encountered in ink jet printing processes is edge raggedness (mid frequency line edge noise, referred to as MFLEN) of the printed image on the substrate, especially when the substrate is plain paper.
Accordingly, while known compositions and processes are suitable for their intended purposes, a need remains for improved ink compositions. In addition, a need remains for ink compositions which exhibit reduced stitch mottle when used in ink jet printing processes. Further, a need remains for ink compositions containing pigment colorants which exhibit reduced stitch mottle when used in ink jet printing processes. Additionally, a need remains for ink compositions which exhibit reduced wet smear when applied to a substrate to form an image. There is also a need for ink compositions which exhibit reduced wet smear when used in ink jet printing processes. In addition, there is a need for ink compositions containing pigment colorants which exhibit reduced smear when used in ink jet printing processes. Further, there is a need for ink compositions which exhibit reduced edge raggedness (MFLEN) when used in ink jet printing processes.